1. Field of the Invention
The present invention relates to, a ceramic heater used for heating a semiconductor wafer in semiconductor production process or for heating a substrate when a thin film is formed according to chemical vapor deposition method or sputtering method, and a method for producing the ceramic heater.
2. Description of the Related Art
As a ceramic heater used for heating a semiconductor wafer in semiconductor production process or for heating a substrate when a thin film is formed according to chemical vapor deposition method or sputtering method, there has been used a heater having a structure in which a heating element consisting of metal foil or rolled circuit or a heating element formed by screen-printing a conductive paste containing metal particles or conductive ceramic particles is buried in a supporting substrate made of sintered body such as silicon nitride, aluminum nitride, or boron nitride (see, Japanese Patent Laid-open (Kokai) No. 2004-220966; and Japanese Patent Laid-open (Kokai) No. 2004-253799).
However, in the case of forming a heater pattern by screen-printing, film-thickness of the heating element easily becomes non uniform, and therefore, occasionally, the heater does not have good heating uniformity. Moreover, there is possibility that organic matter contained in paste used for screen-printing or component of a sintering auxiliary agent contained in a ceramic sintered body becomes a source origin of impurity.
On the other hand, if a conductive layer made of pyrolytic graphite is made by chemical vapor deposition method on a supporting substrate made of pyrolytic boron nitride generated by chemical vapor deposition method and a desired heater pattern is formed by machining therein and furthermore the heater pattern is coated with a coating layer made of pyrolytic boron nitride according to chemical vapor deposition method, a conductive layer having a uniform film-thickness can be easily obtained and a ceramic heater having good heating uniformity can be provided (see, Japanese Patent No. 3560456).
Moreover, because all of the supporting substrate, the conductive layer, and the coating layer, are produced by chemical vapor deposition method, they have higher purity than ones produced by sintering method and have an advantage of a semiconductor wafer being difficult to be contaminated with impurities and are advantageous in heating process.
In the heater in which a conductive layer made of pyrolytic graphite is made by chemical vapor deposition method on a supporting substrate made of pyrolytic boron nitride generated by chemical vapor deposition method and a desired heater pattern is formed by machining therein and furthermore the heater pattern is coated with a coating layer made of pyrolytic boron nitride according to chemical vapor deposition method, through-holes are provided on both ends of the heater pattern and the coating layer at periphery of the through-hole is removed to expose the conductive layer, and a conductive wire from a power source is fixed to the exposed part of the conductive layer by a bolt and a nut that are made of metal or carbon material such as graphite, carbon, or carbon complex material, and thereby the heater is connected to the power source.
However, according to the above connection method, the conductive layer being exposed at the part connected to the power source is wasted and causes abnormal heat generation, and if it is more wasted, electric discharge is caused and the heater becomes damaged. Therefore, there is a problem that use conditions of the heater (heating temperature, atmosphere) are limited. Moreover, in the case that a bolt and a nut that are made of graphite, carbon, carbon complex material, or the like, are used, they become a source origin of particles. Moreover, in the case that a bolt and a nut that are made of metal, particles are difficult to be generated in some time period from an initiation of use, compared to the case that a bolt and a nut that are made of graphite, carbon, carbon complex material, or the like. However, if they continue to be used for a long period, they are degraded with heat even if they are a bolt and a nut made of metal. Ultimately, there is a problem that they become source origins of particles. Also, there is a risk of causing metal contamination of a semiconductor wafer to be treated.
For solving these problems, a ceramic heater described in Japanese Patent No. 2702609 can be exemplified. This is a ceramic heater having a structure in which a heater main body is provided with a heater pattern made of pyrolytic graphite in a substrate made of pyrolytic boron nitride and through-holes are provided in contact ends located in the both ends of the heater pattern and graphite rod members having a predetermined length are fixed through the through-holes using graphite screws so as to be located in the face opposite to the heater patter and then the heater main body and the graphite screws and graphite rod members are integrally coated with a coating layer made of pyrolytic boron nitride.
And, for strengthening mechanically and electrically the attachment of the graphite screws and the graphite rod members, flexible graphite washers are placed between the graphite screw and the heater main body and between the heater main body and the graphite rod member. At the other end of the graphite rod member opposite to the end fixed to the heater main body by the graphite screw, the coating layer made of pyrolytic boron nitride is not formed and a conductive wire is connected to this part.
In the heater described in Japanese Patent No. 2702609, the conductive layer made of pyrolytic graphite to form the heater pattern, the graphite screws, and the members made of carbon material such as graphite rod members are almost entirely coated with a coating layer made of pyrolytic boron nitride. Therefore, it becomes a heater available even in an atmosphere having reactivity with carbon, and generation of particles from the graphite screws, the graphite rod members, or the like, can be suppressed.
In the other end of the graphite rod member opposite to the end fixed to the heater main body by the graphite screw, the coating layer made of pyrolytic boron nitride is not formed and a conductive wire is connected to this part. However, because this part is apart from the heater pattern by the distance of the length of the graphite rod member having the predetermined length, temperature thereof is suppressed to be low. Accordingly, if the heater is used in an atmosphere having reactivity with carbon, degradation thereof is small to some extent. Moreover, if the screw made of metal is used for the connection of the conductive wire, the metal screw hardly becomes a source origin of particles by degradation with heat because the temperature is low.
Here, as main methods for heating a semiconductor wafer with a ceramic heater, there are a method for heating the semiconductor wafer with radiant light from the heater without contact between the wafer and the heater, and a method for heating the semiconductor wafer by heat conduction with putting directly the wafer on the heater.
In the case of performing the radiant heating under a reduced pressure, as use time thereof becomes longer, the heater surface is contaminated by film adhesion due to wrap-around of process gas or by adhesion of scattered things from the peripheral members. Therefore, radiation rate is changed and it occasionally become impossible that a semiconductor wafer is heated in the same manner even with the same electric power. Such a phenomenon is particularly significant in a high-temperature process of 1000° C. or more.
In the case of the heating by directly putting, there are not such problems. Moreover, the heating by directly putting is better in heating efficiency than the radiant heating. Therefore, the heating by directly putting is more appropriate in cost in a high-temperature process.
In the heater described in Japanese Patent No. 2702609, the head of the graphite screw for fixing the graphite rod member sticks out of the heating surface of the heater. Therefore, in the case of putting directly an object to be heated on the heater and heating it, positions in which graphite rod members are provided have to be necessarily outside the region on the heater on which an object to be heated is put. Therefore, there is a problem that the heater becomes large in size.
Moreover, as diameter of a semiconductor wafer has become enlarged in recent years, a large heater has become used as a heater for heating such a wafer. However, in order that current value and power voltage value are made not to be too large or that temperature distribution of the semiconductor wafer is made to be improved, there is frequently used a heater having a two-zone system in which a first heating region in the vicinity of the heater center to be heated by a first power source and a second heating region in the outside thereof to be heated by a second power source are provided and the heater is heated by two power sources.
In the case that a semiconductor wafer is directly put and heated on the heater having a two-zone system in the heater having the structure described in Japanese Patent No. 2702609, the head of the graphite screw sticks out of the heating surface of the heater with regard to the shape of the first heating region and the second heating region of the heater. Therefore, there is no other choice but the shape is made to be one shown in FIG. 6.
The semiconductor wafer is put on the region inside the dashed line 1 in FIG. 6. The heater center is the first heating region 2 and the outside thereof is the second heating region 3. Graphite rod members 4 connected to the first heating region 2 and graphite rod members 5 connected to the second heating region 3 are respectively provided in the most peripheral part of the heater. That is, because the graphite rod members 4 connected to the first heating region 2 located in the vicinity of the heater center are provided in the most peripheral part of the heater, the conductive pathways 6 connecting the first heating region 2 and the graphite rod members 4 have to be provided in the second heating region 3.
Therefore, in the second heating region 3 to be heated by the second power source, there is a heating element to be heated by the first power source. Depending on electric power balance of the first power source and the second power source, the conductive pathway 6 becomes a local heating part or a local low-temperature part. Therefore, there is a problem that temperature distribution of the wafer is adversely affected thereby.
Moreover, the graphite rod members are fixed in a shape of putting the heater main body therebetween through graphite screws. However, a coefficient of linear expansion of graphite is generally 3-7×10−6, and on the other hand, a coefficient of linear expansion in the thickness direction of pyrolytic boron nitride, which is the substrate, is 5-8×10−5. Therefore, stress due to thermal expansion of pyrolytic boron nitride, which is the substrate, acts on the graphite screws or the graphite rod members at each time when temperature of the heater is raised.
Therefore, in the case that the graphite screws and the graphite rod members have small diameters, there is a problem that the head of the graphite screw is damaged or ridge parts of the graphite screw and the graphite rod member are broken down, and therefore, the graphite screw and the conductive layer come to have poor connection and it is feared that the heater comes not to be heated.
If the graphite screw and the graphite rod member have large diameters, damage in the head of the graphite screw or poor connection due to breaking down of the screw can be prevented. However, there is a problem that because cross-section area of the graphite screw or the graphite rod member becomes large, amount of heat to outflow to the outside becomes large and it becomes difficult to heat uniformly an object to be heated.